1. Field of the Invention
The present invention is related to an oscillation circuit, a current generation circuit, and an oscillation method.
2. Description of the Related Art
The precision of the frequency of a clock output from an oscillation circuit is an example of an important parameter in oscillation circuit design. In order to increase the precision of the output clock frequency, sometimes a reduction in the temperature dependency of a reference current generation circuit employed within an oscillation circuit is demanded.
A known oscillation circuit employing a reference current generation circuit is described in paragraphs [0019] to [0036] and FIG. 1 of Japanese Patent Application Laid-Open (JP-A) No. 2008-252414. Specifically, an oscillation circuit 1 described in JP-A No. 2008-252414 is an oscillation circuit installed in a microcontroller, and includes: a reference resistor RE1 that generates a reference current; an operational amplifier AMP1 that supplies current to the reference resistor RE1; a reference voltage generation circuit 24 that determines a reference voltage applied to the reference resistor RE1; an integrated circuit 2 that defines an oscillation frequency based on bias potentials PB1, NB1 generated by the reference current and constant currents Ip, generated by the bias potentials PB1, NB1; and a setting resistor 25 that sets temperature dependency of the output voltage of the reference voltage generation circuit 24 so as to be the same temperature dependency as the temperature dependency of the reference resistor RE1. The oscillation circuit 1 of JP-A No. 2008-252414 purports to obtain an oscillation frequency with low temperature dependency based on the bias potentials that are temperature independent, and the constant current.
JP-A No. 2011-135349 describes another example in which an oscillation circuit employs a reference current generation circuit. In the reference current generation circuit of the oscillation circuit described in JP-A No. 2011-135349, a current with positive dependency on absolute temperature, and a current with negative dependency on absolute temperature are added together to generate a current that is substantially temperature independent (paragraphs [0052]-[0065], FIG. 8).
However, the reference current generation circuit 24 for generating a reference current described in JP-A No. 2008-252414 has a circuit configuration employing an operational amplifier, and therefore is difficult to operate at low voltages. Specifically, the reference voltage generation circuit 24 described in JP-A No. 2008-252414 employs a band gap circuit employing an operational amplifier as illustrated in FIG. 5 of JP-A No. 2008-252414.
Operational amplifiers are generally configured by at least three vertically stacked Metal Oxide Semiconductor Field Effect Transistors (MOSFET, also referred to as MOS transistors below) between the power source and GND (ground, earth). Accordingly, a power source voltage of 1.8V or greater is required for a MOS transistor with a threshold value voltage of 0.6V. Further, there are also i an increase in the number of elements and an increase in the layout surface area due to employing dedicated band gap circuits and plural operational amplifiers, as well as requiring a MOS transistor with high withstand voltage. Similar points apply in the reference current generation circuit described in JP-A No. 2011-135349.